The present invention relates to a catalyst which contains iridium and at least one other doping element on a support for preparing substituted aromatic amines by hydrogenation of the corresponding substituted nitroaromatic compounds. In another aspect, the invention relates to the method of making the catalyst.
The hydrogenation of aromatic nitro compounds with halogen substituents, in particular chlorine, to prepare the corresponding amines has been known for some time (Ullmann, Enzyklopadie der technischen Chemie, 5th edition, vol. A2, page 46 (1985)) and is performed in the presence of metal catalysts using hydrogen as reducing agent. A serious problem in the reaction is the undesired dechlorination of the aromatic compounds, which leads to a reduction in the yield of aromatic amine compound. For this reason, many patents have dealt with processes which attempt to keep the elimination of halogens as low as possible.
Thus, the use of platinum and ruthenium on support materials as shown in U.S. Pat. No. 4,760,187 can lead to an improvement in selectivity, as does the use of platinum, palladium, rhodium, iridium, ruthenium and osmium-containing catalysts which are after-treated with acidic phosphorus compounds (DE-OS 30 06 748).
The activity and selectivity of noble metal catalysts can be affected by the presence of a co-catalyst in the reaction mixture. According to U.S. Pat. No. 5,105,012 iron powder or an iron salt can be added to the reaction mixture as a co-catalyst in addition to palladium on a carbon support as the main catalyst. U.S. Pat. No. 3,253,039 suggests adding silver nitrate to the reaction mixture in addition to a catalyst consisting of platinum on carbon. Silver is then distributed in the entire reaction mixture without being alloyed with the platinum in the catalyst. Lead, copper, nickel, bismuth and chromium nitrate have also been tried as heavy metal additives.
The activity and selectivity of hydrogenation can also be affected by specific doping of a noble metal catalyst with a variety of A and B group elements. Thus, for example, DE 42 36 203 A1 suggests using a platinum catalyst on active carbon doped with nickel and/or cobalt. According to DE 42 18 866 C1, the selectivity of hydrogenation is improved by doping a platinum catalyst on active carbon with copper. In this case, platinum and copper are deposited onto the active carbon support at the same time and then reduced.
Known hydrogenation catalysts are prepared, for example, by introducing the support material into a noble metal salt solution and evaporating the solvent. Optionally, the catalyst is then reduced. As an alternative to this, the support material may also be impregnated with the catalytically active elements by placing the support material in contact with a solution of these elements and precipitating the hydroxides of these elements in an alkaline medium. A reduction procedure may also follow this. Furthermore, it is known that a solution of the noble metal salts may be sprayed onto the support material.
The hitherto known processes for hydrogenation of substituted aromatic nitro compounds, however, still present some problems relating to activity and selectivity. Thus, the use of modifiers or promoters in combination with nitrogen-containing additives during hydrogenation leads to halogenated azobenzene and azoxybenzene derivatives (EP-OS 0 073 105). The formation of such compounds as side-products during the hydrogenation of halogenated nitroaromatic compounds should be avoided due to their toxicity.
An object of the present invention is to provide a catalyst for the hydrogenation of substituted nitroaromatic compounds which is distinguished in particular by improved selectivity as compared with known catalysts.